Out-of-plane MEMS gyroscopes (OPG's) are used to determine angular rotation by measuring Coriolis forces exerted on a resonating proof mass. A typical out-of-plane MEMS gyroscope includes two silicon proof masses mechanically coupled to a substrate, typically glass, using one or more silicon suspension springs. A number of recesses etched into the substrate allow selective portions of the silicon structure to move back and forth freely within an interior portion of the device. In certain designs, substrates can be provided above and below the silicon structure to sandwich the proof masses between the two substrates. A pattern of metal traces formed on the substrate(s) can be used to deliver various electrical bias voltages and signal outputs to the device.
The drive system for many MEMS gyroscopes typically includes a number of drive elements that cause the proof mass to oscillate back and forth along a drive axis perpendicular to the direction in which Coriolis forces are sensed. In certain designs, for example, the drive elements may include a number of interdigitated vertical comb fingers configured to convert electrical energy into mechanical energy using electrostatic actuation. Such drive elements are described, for example, in U.S. Pat. No. 5,025,346 to Tang et al., which is incorporated herein by reference in its entirety. In an out-of-plane MEMS gyroscope, these interdigitated comb fingers can produce a large bias, or false angular rate output signal. This bias can be caused by the large electric field in the gap between adjacent comb fingers, and from the fact that the electric field is directed along the sense axis of the gyroscope (i.e. the axis along which the Coriolis force acts). Thus, a force is produced along the sense axis when the movable comb fingers are not perfectly centered between the adjacent stationary comb fingers. This force produces a displacement of the gyroscope proof mass along the sense axis, which is indistinguishable from the displacement along the sense axis produced by the Coriolis force in the presence of angular rotation of the gyroscope. Forces produced by the interdigitated drive comb fingers along the sense axis can also change the capacitance of the sense comb electrodes used to detect the displacement produced by the Coriolis force, thus changing the output signal of the gyroscope. As a result, an error is produced in the scale factor, defined as the gyroscope output signal per unit rotation rate.